Hydrogen production is a high concern in the fields of energy, refining and petrochemical industry. Hydrogen can be used for example as fuel for fuel cells and for the production of energy by combustion in gas turbines. Hydrogen is also used as a reagent in many refining and petrochemistry processes, as well as for the production of ammonia.
Hydrogen is predominantly produced from hydrocarbon-containing feeds of various origins (natural gas, petroleum cuts, biomass, etc.). The main hydrogen production processes using such feeds are steam reforming, gasification, partial oxidation and autothermal reforming. All these methods allow to produce synthesis gas that predominantly contains hydrogen, carbon monoxide (CO) and carbon dioxide (CO2), as well as methane (CH4) and heavier hydrocarbons and, if air is used in the conversion process, nitrogen and argon or any other molecule initially present in air. Hydrogen therefore has to be purified prior to final use.
Several techniques for purifying hydrogen are known from the prior art: separation through absorption by a chemical solvent (of the amines family for example) or a physical solvent, separation by pressure swing adsorption (PSA) or membrane separation.
The absorption methods using amines do not allow all the impurities to be removed, notably molecules such as nitrogen, argon, carbon monoxide and methane.
The performances of hydrogen purification methods using membranes greatly depend on the membrane material used. Only mixed metals (palladium alloys for example) and oxides allow to obtain CO contents in the final hydrogen comparable to those of the hydrogen produced by PSA. Now, these membranes are very costly and can only work at very high temperatures (above 350° C.), which considerably increases the total hydrogen production cost.
PSA therefore is the most suitable method for producing high-purity hydrogen.
The principle of PSA is well known to the person skilled in the art. The gas to be purified flows, at the high pressure of the cycle referred to as “adsorption pressure”, through adsorbent material layers that preferably retain the impurity (or impurities). These compounds are then extracted from these adsorbents by lowering the pressure down to a pressure referred to as “desorption pressure”. The principle of PSA is to cyclically link together these high-pressure adsorption and low-pressure desorption stages.
The purified hydrogen stream is thus produced at high pressure, whereas the residual impurity-rich stream is produced at low pressure.
PSA performances are evaluated by determining the value of the dynamic capacity of an adsorbent. The dynamic capacity of an adsorbent is defined as the difference between the adsorbed amount under the adsorption stage conditions and the amount that remains adsorbed after desorption. The dynamic capacity depends on the adsorption isotherms of the various feed constituents. Regardless of the kinetic effects, the dynamic capacity of a given constituent can be assessed by calculating the difference between the adsorbed amount at adsorption pressure Pads and the adsorbed amount at desorption pressure Pdes for this constituent (FIG. 1). PSA performances are mainly characterized by the purity of the hydrogen produced, the hydrogen yield (i.e. the amount of hydrogen produced in relation to the amount of hydrogen contained in the feed) and the productivity (the amount of hydrogen produced in relation to the mass of adsorbent contained in the process and in relation to time). These three performance characteristics are connected. For a given dynamic capacity, one of these performances can only be increased to the detriment of another or of the other two. For example, if a certain purity is obtained for a given productivity, the purity is lower for a higher productivity. On the other hand, a dynamic capacity increase can allow to increase one of these performances while maintaining the others constant. A certain purity can be obtained for a given feed flow rate, or a lower purity for a higher feed flow rate.
Conventional PSA for hydrogen purification contains several adsorbent layers. In general, an activated charcoal layer is first used to adsorb the CO2, then a zeolite 5A or NaX layer to adsorb the CH4, the CO and the other impurities possibly present in the feed. When water is present in the feed, the PSA process can also contain a preliminary silica gel layer.
These adsorbents have good adsorption capacities at high pressure for the various impurities contained in the feed. On the other hand, their regeneration (impurity desorption) requires lower pressures close to atmospheric pressure. It can be seen on curve (a) in FIG. 1 that the dynamic capacity of conventional adsorbents drops considerably when the desorption pressure is too high. Consequently, the CO2-rich stream resulting from hydrogen purification by conventional PSA is always produced at low pressure.
Production of a residual stream of impurities at high pressure could have applications for the production of energy (by combustion for example), in the chemical industry (methanol and dimethylether synthesis, supercritical solvent, etc.), in the drilling sphere (enhanced oil recovery) or for CO2 storage (after compression and/or liquefaction), etc.
Several solutions allowing to produce the impurity stream at a higher pressure have been provided in the prior art.
Patent application WO-2006/112,724 describes a method of producing hydrogen for gas turbines, from methane steam reforming. The mixture leaving the water gas shift reactor is separated by PSA to produce a hydrogen-rich stream containing 10-20% impurities (CO, CH4 and CO2) and a residual stream predominantly containing CO2, as well as CO, H2O, H2 and CH4. The method described aims to recover this residual stream at a high pressure and not at atmospheric pressure so as to reduce its recompression cost. No technical solution has however been provided to produce the CO2 at high pressure while maintaining good performances for hydrogen purification.
Patent application WO-2006/097,703 describes a method of producing hydrogen for either the production of energy by combustion or as high-purity fuel, with recovery of the CO2 coproduced. A PSA process is proposed as the preferred separation mode because it allows to reach very high hydrogen purities. The CO2-rich residual gas resulting from PSA is recovered by condensation. In order to reduce the CO2 recompression cost, the method described aims to maintain a high residual gas pressure at the PSA outlet. This desorption pressure increase leads to a dynamic capacity loss and lowers the hydrogen yield. It is therefore necessary to reach a compromise between the hydrogen recovery ratio and the residual gas pressure.
Patent application WO-00/18,680 describes a method of producing hydrogen for gas turbines, combined with separation and sequestration of the CO2 coproduced. The operating pressures of the steam reforming reactor and of the water gas shift reactor are very high. The H2/impurities separation unit is therefore also operated at a very high pressure. The preferred separation mode is CO2 absorption by an amine solution. The amine solution is regenerated at a pressure ranging between 0.5 and 0.8 MPa. The implementation proposed does not allow to produce a high-purity hydrogen stream because the amine solution does not absorb carbon monoxide and methane. The scheme is therefore only suited for the use of hydrogen for gas turbines.
Patent application WO-2003/070,357 describes a PSA process of separating a gaseous mixture containing hydrogen and impurities of hydrocarbon type allowing to produce a high-purity hydrogen stream and a hydrocarbon-rich residual stream. To upgrade the residual stream, it is interesting to obtain it at a high pressure, above 0.4 MPa. The adsorbent used is a combination of an activated charcoal and of a silica gel, which allows to desorb the hydrocarbons at high pressures, but with a very appreciable hydrogen yield loss. Activated charcoal and silica gel are adsorbents that are well known to the person skilled in the art but they do not have a satisfactory dynamic capacity as regards CO2 when desorption is carried out at high pressures.
Analysis of the state of the prior art shows that the advantage of co-producing a CO2 stream at high pressure during hydrogen purification has been identified. Now, the solutions provided do not allow to co-produce CO2 under pressure without a compromise as regards the hydrogen yield and/or purity. They are based on changes in the operating conditions of existing processes, which involve a purity or a hydrogen yield decrease.